Factors Influencing Boll Numbers

The overall growth and development of the cotton plant follows a typical sigmoid curve with the rate of development largely determined by temperature.

It is important to maintain vigorous vegetative growth before flowering to develop the plant structure, leaves, branches and roots that will support and supply resources for developing boll load. Once reproductive structures begin to develop, vegetative and root growth normally slows as plant resources are allocated to developing fruit.

The aim of good crop management is to keep the reproductive and vegetative growth in balance for as long as the season allows, timing cut-out to maximize the number of mature bolls at harvest. The longer the period of fruit production before cut-out generally translates into higher yields.

Plant Population

Cotton is a perennial shrub and is able to adapt to the environment it is grown in. Evenly spaced plant stands help the crop to develop uniformly.

Water Management

This greatly influences boll numbers and yield. Boll number and cotton yields will be reduced by a lack of water as well as from water logging, and stress effects are cumulative. Good irrigation design, bed preparation and irrigation management is required to grow high yielding cotton.

Nutrition

Nitrogen is required throughout the whole growing season but the peak demand for nitrogen is during flowering, with 55% taken uptake during this period. Insufficient nitrogen availability will reduce production of leaves & stems, reducing potential fruiting sites which lowers boll numbers and reduces yield.  Higher yielding crops are requiring greater flux of nitrogen per day as the growing period has not increased greatly but yields have increased.  Peak nitrogen uptake is closer to 5 kg/ha/day in these crops, which is placing larger demands on the soils and the timing of nitrogen fertilizer applications. Higher yielding crops generally have 40-45% of N applied in-crop.

Phosphorus plays an important role in the plant’s production of energy.  A shortage of phosphorus will reduce a plant vigour, which restricts root and foliage growth, delaying and reducing production of fruiting sites and crop maturity.  Under cool conditions, cotton responds to low rates of phosphorus applied with the seed.  Crop removal applications of phosphorus should be well distributed in the hill or bed as cotton responds to good phosphorus nutrition, not bands of phosphorus fertilizer.

Under soil conditions where phosphorus and zinc availability is limited, foliar applications of P and Z have stimulated plant growth requires phosphorus, but phosphorus is relatively immobile in the soil so uptake relies on the crop having a well-developed root system; which requires phosphorus. 

Potassium is generally associated with water regulation through cell turgor pressure and deficiencies reduce efficient use of water.  Cotton requires nearly as much potassium as it does nitrogen.  Plants suffering potassium deficiency during fruiting will have lower number of fruiting sites, bolls and final yield.  Potassium applications to replace crop removal should be well distributed in the hill or bed like phosphorus.

Potassium is generally taken up in luxurious quantities if available and this occur before biomass production.  Uptake (61% of total) and redistribution of K during flowering and boll development is important to satisfy the crops requirement. Foliar applications of potassium can supply shortages in supply.  Foliar applications have shown good crop responses when applied either in-front or tropical rainfall events, which cause water logging with days of cloudy weather or when petiole analysis show low level of potassium.

Sulphur deficiency can decrease biomass production in cotton, which in turn reduces potential fruiting sites and boll numbers.  Sulphur is involved in the synthesis of proteins and is required at a considerable lower amount than nitrogen.  Sulphur removed by a 15 b/ha cotton crop is 15 kg/ha, compared to 150-180 kg/ha for nitrogen.

Zinc is very important for cotton production even through it is only required in small quantities as cotton is mostly grown on alkaline vertisol soil.  Zinc availability was inherently low on these soils but repeated applications has built soil zinc levels. However, cotton still suffers from short term deficiencies generally caused by water logging events.  Zinc deficiencies reduces production of growth hormones such as auxins, which in turn reduces crop growth.  Approximately 73% of total zinc uptake occurs during flowering.

Iron is an essential nutrient required in very small amounts for chlorophyll synthesis and in some enzymes.  Issues generally only arise when crops are suffering from water logging in alkaline soils. Deficiency symptoms disappear quickly once the soil beginning to aerate.  Foliar applications of iron pre or post waterlogging can be used to manage the crop.

Deficiencies in Calcium, Magnesium, Boron, Manganese and Molybdenum has been shown to reduce bolls numbers but are generally not an issue in Australian cotton growing areas.